Powder coating composition for electrodeposition

ABSTRACT

A powder coating composition for electrodeposition which comprises powdery particles having 0.1 to 50 microns particle size wetted with an organic solvent having a boiling point of 50° to 250° C in a range of 5 to 100 parts by weight per 100 parts by weight of the powdery particles in said powder coating composition and dispersed in an aqueous medium containing a surface active agent of 0.05 to 5 parts by weight per 100 parts by weight of the powdery particles in said powder coating composition to be dispersed, in which said organic solvent does not dissolve the organic high molecular weight compound in said powder coating composition and is scarcely soluble with said aqueous medium which is water or a mixture of water and water soluble organic solvents.

This is a division of application Ser. No. 465,078, filed Apr. 29, 1974, now U.S. Pat. No. 3,960,791; which was a division of application Ser. No. 211,660, filed Dec. 23, 1971, now abandoned.

This invention relates to a powder coating composition for an electrodeposition.

Further, the present invention relates to a powder coating composition for the electrodeposition which comprises an aqueous dispersion of fine powder of organic high molecular weight compounds.

Still further, the present invention relates to a powder coating composition for electrodeposition which is excellent in throwing power, that is, by using the powder coating composition of the invention in electrophoretic coating processes, uniform coatings can be formed even on the reverse surface of an anode which is not opposite to each electrode, and the surface of the anode which is opposite to each electrode.

The powder coating composition for electrodeposition which is prepared by suspending the powdery organic high molecular weight compounds in water, organic solvents of mixtures thereof, is heretofore generally known. However, they are inferior with respect to the throwing power as compared with the electrodeposition coatings of the water soluble or dispersible coating composition in the prior art. One of the disadvantages in the throwing power, that is, the inferior formation of the coating on the reverse surface not opposite to each electrode or on the surface in a complicated shape, is considered to be caused by the fact that the electric resistance of the deposited coating cannot be increased owing to the existence of water and other electroconductive materials between the deposited powdery coating particles. Therefore, the powder coating composition could not be practically employed on account of its inferior throwing power in electrophoretic coating processes However, varieties of organic high molecular weight compounds can be used for all their several characteristic advantages; a coating film having more than 100 microns in thickness, in some cases 300 microns in thickness, can be formed, whereas in case of electrodeposition of water soluble or dispersible materials, a coating film of only 50 microns in thickness at the most is formed; the time required for the electrodeposition is reduced; and the obtained coating is superior in water resistance because it is not necessary to introduce hydrophobic groups in the organic high molecular weight compound which are required in the electrodeposition of water soluble or dispersible materials of the prior art.

The term "throwing power" herein used means the degree of the formation of the electrodeposited coating film on the front and base surfaces of an article to be coated. When differences between the thickness of the electrodeposited coating film on the surface which is opposite to each electrode and of the electrodeposited coating film on the reverse surface is small, or when the electrodeposited coating film on this reverse surface of the article is uniformly formed, throwing power is good or excellent.

As a result of the inventors' eager investigation in order to eliminate the aforementioned defects, it was found that the throwing power of the powder coating composition for the electrodeposition can be improved very much by wetting the fine powdery particles consisting of the organic high molecular weight compounds as a main component with using a certain organic solvent, and in addition a smooth coating film can be obtained by backing the deposited layer and the formed coating film is excellent both in mechanical and chemical properties.

A powder coating composition for electrodeposition in the present invention which comprises powdery particles for a powder coating composition having 0.1 to 50 microns particle size wetted with organic solvent having a boiling point of 50 to 250° C in a range of 5 to 100 parts by weight per 100 parts by weight of the powdery particles in said powder coating composition and dispersed in an aqueous medium containing a surface active agent of 0.05 to 5 parts by weight per 100 parts by weight of the powdery particles in said powder coating composition to be dispersed, in which said organic solvent does not dissolve the organic high molecular weight compound in said powder coating composition and the powder is scarcely soluble with said aqueous medium which is water or a mixture of water and water soluble organic solvents. That is, the particles of said organic compounds are wetted with the organic solvents, and then the particles are suspended in the aqueous medium containing surface active agent to form the powder coating composition of the present invention. In the composition of the present invention, the characteristic advantages of the powder coating composition for the electrodeposition are maintained, while the throwing power of the composition is improved. Thus the difficulties in the electrophoretic coating of the powder coating composition for practical uses have been overcome.

As the organic high molecular compounds being used in the present invention, resins which are ordinarily used for the powder coating composition can be utilized. For example, polyolefine, epoxy resin, polyvinyl chloride, polyester, acrylic resin, and polyvinylidene fluoride are typical. These resins are pulverized into 0.1 to 50 microns particle size, and particles in the range of 1 to 20 microns are preferable for practical uses. It is difficult to obtain particles of less than 0.1 micron in diameter through the usual method, and in case of particles of more than 50 microns in diameter, it is difficult to keep the electrodepositing bath in the state of a stable suspension and a precipitate of the particles is liable to form, and, in addition, the thickness of the formed coating film becomes unnecessarily large. Therefore the particle size in the aforementioned range is most suitable. Further, as pigments and other additivies to be mixed into said organic high molecular weight compounds, the ordinarily known additives such as pigments including extender pigments and coloring pigments may be used.

As organic solvents to wet the particles for the powder coating composition of the present invention, which do not dissolve the powdery particles for the powder coating composition and are nearly insoluble with the aqueous medium, those having a boiling point of 50° to 250° C can be used. For example, hydrocarbon (hydrocarbon halides and nitro-hydrocarbons being included), alcohol, ketone, ester and ether solvents can be used, however, each must be used properly according to the kind of organic high molecular weight compound used. For example, for powdery particles using epoxy resin as a vehicle, one or a mixture of hydrocarbon solvents such as hexane, heptane, isooctane, mineral spirits, naphtha, cyclohexane, tetrahydronaphthalene, decahydronaphthalene and kerosene can be used; for the powdery particles using polyvinyl chloride as a vehicle, one or a mixture of hydrocarbon solvents such as toluene, xylene, cyclohexane, tetrahydronaphthalene and decahydronaphthalene, hydrocarbon halide solvent such as trichloroethylene, and alcohols such as amyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol and cyclohexanol can be used; for powdery particles using polyester as a vehicle, one or a mixture of hydrocarbon solvents such as hexane, heptane, isooctane, mineral spirits, naphtha, and kerosene, and alcohols such as amyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol, and cyclohexanol can be used; for powdery particles using acrylic resins as a vehicle, one or a mixture of hydrocarbon solvents such as hexane, heptane, isooctane, mineral spirits, naphtha, cyclohexane, tetrahydronaphthalene, decahydronaphthalene and kerosene can be used.

Further, for powdery particles using polyolefin as a vehicle, one or a mixture of hydrocarbon solvents such as benzene, toluene, xylene, tetrahydronaphthalene, decahydronaphthalene and hydrocarbon mixture solvent, ketones such as methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone and isophorone, esters such as ethyl acetate, butyl acetate, octyl acetate, amyl acetate, methyl amyl acetate and Cellosolve acetate, and alcohols such as amyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol and cyclohexanol can be used; for powdery particles using polyvinylidene fluoride as a vehicle, one or a mixture of hydrocarbon solvents such as benzene, toluene, o-dichlorobenzene, n-heptane, isooctane, kerosene and turpentine, and esters such as ethyl acetate and butyl acetate can be used

The amount of the organic solvent to be used is 5 to 100 parts by weight per 100 parts by weight of the powdery particles in the powder coating composition, and the range of 20 to 60 parts by weight of the solvent is most preferable. When the amount of the organic solvent is less than 5 parts by weight, the prominent effect in improvement of the throwing power, which is the principal object of the invention, cannot be achieved. When the amount is more than 100 parts by weight of solvent being used, a coating film having a sufficient thickness cannot be obtained for a certain supply of the electric power because the content of the organic solvent becomes larger in the deposited film, as compared with the case in which the organic solvent of less than 100 parts by weight is used. Further, since a larger amount of the surface active agent must be used in order to disperse an organic solvent of more than 100 parts by weight into the aqueous medium, the coated film obtained has the faults of discoloration, pin-holes and craters.

In a suspension consisting of only a mixture of the particles, water soluble organic solvent and aqueous medium, the particles in the suspension are moved toward the electrode which has the charge opposite to the charge of said particles when the electric field is imposed on the bath, however, the velocity of the electrodeposition is low, and the adherence of the particles to the electrode is very weak. Therefore, the deposited film cannot be obtained since the deposited particles are peeled off from the article when this article (electrode) is lifted out of the bath. Further, an organic solvent which mixes with water is scarcely dissolved in water, and therefore such suspension cannot be used for the electrodeposition of the powder coating composition for the above-mentioned reasons because water existing between the deposited powdery coating particles is electroconductive and as a result, throwing power is lowered. Accordingly, in the present invention, surface active agents are used in order to give the electric charge to the wetted particles, and to facilitate the dispersion of the particles in the aqueous medium, and thereby the electrodeposition coating can be proceeded smoothly. The amount of the surface active agent to be used may be in the range of 0.5 to 5 parts by weight per 100 parts by weight of the powdery particles in the powder coating composition. If the amount of the surface active agent is less than 0.05 parts by weight, the particles of the powder coating composition cannot be dispersed satisfactorily into the aqueous medium, and more than 5 parts by weight of the agent is not desirable because it causes faults in the coating such as discoloration, craters and pin-holes. As surface active agents, the anionic active agents and nonionic active agents can be used, however, it is not preferable that the cationic active agents be used. As anionic surface active agents, one or a mixture of fatty acid alkali metal salts having from 8 to 22 carbon atoms, alkylsulfates in which the alkyl group has from 8 to 18 carbon atoms, alkylsulfonates in which the alkyl group has from 8 to 18 carbon atoms, alkylarylsulfonates in which the alkyl group has from 8 to 18 carbon atoms and polyethylene glycol alkyl ether sulfonates in which polyethylene is lower than hexaethylene and the alkyl group is from 8 to 18 carbon atoms can be used. As nonionic surface active agents, one or a mixture of polyoxyethylene alkyl ethers in which there are from 8 to 15 oxyethylene units and the alkyl group is from 8 to 18 carbon atoms and polyoxyethylene alkyl esters in which there are from 8 to 15 oxyethylene units and the alkyl group is from 8 to 18 carbon atoms may be used. In use, the surface active agent can be added previously to the aqueous medium or be added afterward.

The aqueous medium of the present invention is water only or a mixture of water and water soluble organic solvents. The water soluble organic solvents are of use for the dispersion of the powder coating composition. As for such solvents, alcohols, ethers, ketones and esters may be exemplified. For example, one or a mixture of alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol and propylene glycol; ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether and ethylene glycol monobutyl ether; ketones such as acetone, diacetone and methylethyl ketone; and esters such as ethyl acetate, propyl acetate, methyl acetate, amyl acetate, methyl acetate and methyl carbitol acetate, can be used. The water soluble organic solvents are used alone or as a mixture in the range of less than 90% by weight.

In the present invention, the conditions for the electrophoretic deposition are as follows:

Concentration of the composition in the bath: ca. 5 - 20% by weight

Bath temperature: ca. 15° - 40° C

Applied voltage: 100 - 400 volts.

In operation, when good throwing power is desired, supply of a relatively higher voltage is preferred.

As compared with the ordinary electrodeposition bath using the ordinary powder coating material, the bath using the powder coating composition of the present invention has excellent throwing power, the reason for which is considered to be that the electrical resistance of the deposited film becomes large by the use of the powdery particles wetted with the organic solvent. This is, the particles which are wetted with the organic solvent are deposited on the surface of the article to be coated as the anode to form a coating film. In the course of the electodeposition of the coating film, the organic solvent which the particles contacts and which unite one with another, acts to exclude the water contained as the aqueous medium from the deposited film. Therefore, the electric conductivity between the deposition bath and the article to be coated is lost, the electric resistance of the deposited film is increased, the wetted particles are electrodeposited on the part of article as an anode which is yet electrodeposited, and, thus, the throwing power can be improved. The water content of the deposited film is thereby decreased, and accordingly, the particles of the coating material can be smoothly and densely deposited on the surface of the article to form a desirable coating film. The organic solvent used is volatile so that it can be evaporated by leaving the treated article at the room temperature or by heating the article at a temperature of 50° to 150° C. The dried article is then baked to form a finished coating which has excellent physical and chemical properties and has no undersirable defects such as discoloration, craters and pin-holes. The coating film obtained in the present invention is likewise good in the finished surface as compared with those obtained by the other powder coating method such as electrostatic coating method, and therefore, the characteristic of the powder coating method can be fully attained. Further, in the prior art, it is believed that the coating obtained through electrodeposition is inferior in water resistance, while the powder coating composition obtained by the present invention is superior in water resistance as the water content thereof is very low. Still further, in the ordinary methods, the electrodeposition bath must always be stirred to prevent the particles from precipitation when particles of more than 5 microns in diameter are used. However, in the method of the present invention, the particles of the composition do not easily precipitate because the gross specific gravity of the particles is decreased by using an organic solvent which is less than 1.00 in the specific gravity, and accordingly, the control of the electrodeposition bath is very simple.

The composition of the present invention has an excellent throwing power, and therefore, it can be utilized for the coating of articles having complicated shapes or concave portions which have not been applied with good coatings by ordinary methods including electrodeposition coating. That is, the composition of the present invention can be used for the coatings of, for example, sheets, plates, steel bars such as C-shape steel and H-shape steel, steel pipes, joints, elbows, parts and accessories of machines and vehicles, and structural steels.

In the following, the objects and features of the present invention will be explained further by means of preparation examples and coating examples, in which parts of percent means parts by weight of percent by weight, unless otherwise indicated.

PREPARATION EXAMPLE 1 Preparation of Epoxy Resin Powder

The following materials were weighed and mixed well by fusing at 90° to 140° C for 10 minutes.

    ______________________________________                                         Mixed Materials:     Epikote No. 1001 (a trade name, an epoxy resin      made by Shell Chemical Corporation)                                100    parts     Dicyandiamide (a hardener) 6      parts     Rutile type titanium white (a pigment)                                20     parts     Bultvar B-79 (made by Monsanto Co.)     as antisagging agent                                4      parts     Total:                     130    parts     ______________________________________                                    

After the mixing, the mixture was cooled and pulverized such that the average particle size of the obtained powder was 10 to 20 microns. The composition of the present invention must be finely pulverized as compared with the powder for the ordinary electrostatic coating (from 50 to 200 microns). The same shall apply to the following Preparation Example 2.

PREPARATION EXAMPLE 2 Preparation of Polyester Resin Powder

The following materials were weighed and mixed well by fusing at 80° to 130° C for 20 minutes.

    ______________________________________                                         Mixed Materials:                                                               Ester Resin No. 20 (a trade name, a thermoplastic                              polyester resin consisting of terephthalic acid,                               isophthalic acid, ethylene glycol and neopentyl glycol made                    by Toyobo Co., Ltd., Japan)                                                                               80     parts                                        Epikote No. 1004 (a trade name, an epoxy                                       resin made                                                                     by Shell Chemical Corporation)                                                                            20     parts                                        Red iron oxide (a pigment) 20     parts                                        Modaflow (made by Monsanto Co.) as antisagging                                 agent                      3      parts                                        Total:                     123    parts                                        ______________________________________                                    

After the mixing, the obtained mixture was cooled and pulverized into fine particles, the average particle size of which was 10 to 20 microns.

PREPARATION EXAMPLE 3 Preparation of Acrylic Resin Powder

The following materials were weighed and mixed well by fusing at 80° to 139° C for 20 minutes.

    ______________________________________                                         Mixed Materials:                                                               Acrylic resin for powder coating.sup.*1                                                                   100    parts                                        Adipic acid                6      parts                                        Titanium dioxide (rutile type)                                                                            30     parts                                        Modaflow (as antisagging agent)                                                                           1      part                                         Total:                     137    parts                                        ______________________________________                                          .sup.*1 Granular solid copolymer resin consisting of styrene : methyl          methacrylate : butyl acrylate : glycidyl methacrylate = 40 : 20 : 30 : 10      % by weight.                                                             

After the mixing, the obtained mixture was cooled and pulverized into fine particles, the average particle size of which was 15 to 25 microns.

COATING EXAMPLE 1 Preparation of Electrodeposition Bath

The epoxy resin powder, in an amount of 200 parts, prepared in Preparation Example 1 was wetted uniformly by dropping gradually 100 parts of mineral spirits therein.

The thus obtained wetted epoxy resin powder was suspended with stirring into an aqueous solution consisting of 1800 parts of de-ionized water and 2 parts (as solid) of potassium oleate soap.

Electrodeposition

The suspension thus obtained was put in a vessel made of polyvinyl chloride (length x width x depth: 100 × 150 × 200 mm), and a cold-finished steel plate (100 × 200 mm) was placed near the longer wall on one side and charged negatively. A zinc phosphate treated steel plate (70 × 150 × 0.8 mm) was placed 120 mm apart from said negatively charged plate, and was charged positively. Thereby, the electrodeposition coating was carried out with an electric voltage supply of 150 volts for 3 minutes.

Drying and Baking

The electrodeposited plate was dried for 20 minutes in a hot air dryer at 80° C to remove the mineral spirits and water from the deposited film, and then the plate was baked at 200° C for 15 minutes.

Thickness of Coating Film

The thickness of the coating film on the surface which faced to the electrode was 120 microns, and the thickness of the film on the reverse surface was 110 microns. Therefore, it will be understood that the throwing power of the composition of the present invention is excellent.

Comparison Test

For comparison, another electrodeposition coating and baking were carried out in the same manner as the above steps except that the powder of epoxy resin was not wetted by mineral spirits. The dried thickness of the obtained coating film on the surface which faced to the electrode was 180 microns, while on the reverse side of the plate, the peripheral 1 cm range was covered with a thin coating and a coating film was not formed at all on the central portion of the plate.

COATING EXAMPLE 2 Preparation of Electrodeposition Bath

The polyester resin powder, in an amount of 200 parts, prepared in Preparation Example 2 was wetted uniformly by dropping slowly 40 parts of hexane therein.

The thus obtained wetted polyester powder was suspended with stirring in an aqueous solution consisting of 1700 parts of de-ionized water, 100 parts of n-butyl alcohol and 2 parts (as solid) of potassium oleate soap.

Electrodeposition, Drying and Baking

The electrodeposition, drying and baking were carried out in the same manner as the foregoing Coating Example 1, except that the electric voltage was supplied for 2 minutes, the baking was at 180° C for 30 minutes.

Result

The thicknesses of the coating film as obtained was 70 microns on the electrode side and 65 microns on the reverse side which showed the excellent throwing power. The surface of the coating film was smooth and glossy.

Comparison Test

For comparison, another electrodeposition coating and baking were carried out in the same manner as the above steps except that the powder of polyester resin was not wetted by hexane. The dried thickness of the obtained coating film on the electrode side surface was 90 microns, while almost no coating was formed on the reverse surface.

COATING EXAMPLE 3 Preparation of electrodeposition Bath

The acrylic resin powder, in an amount of 200 parts, prepared in Preparation Examples was wetted uniformly by dropping gradually 100 parts of Isopar E (a mixture of aliphatic hydrocarbon solvent made by Esso) therein.

The thus obtained wetted acrylic resin powder was suspended with stirring in an aqueous solution consisting of 1800 parts of de-ionized water and 3 parts (as solid) of Demol EP (polycarboxylic acid type anionic surface active agent made by KAO-ATLAS Co., Ltd., Japan).

Electrodeposition, Drying and Baking

The electrodeposition drying and baking were carried out in the same manner as the foregoing Coating Example 1, except that electric voltage supplied was 100 volts for 3 minutes, the baking was at 200° C for 25 minutes. The thickness of the coating film obtained was 105 microns on the electrode side and 95 microns on the reverse side.

COATING EXAMPLE 4 - 7

The preparations of electrodeposition bath were carried out in the same manner as the foregoing Coating Example 1, except that the ratio of the epoxy resin powder, mineral spirits and potassium oleate soap which are represented by weight parts was as in the following Table 1.

                  TABLE 1                                                          ______________________________________                                         Coating Example No.  4      5      6    7.sup.*1                               ______________________________________                                         The epoxy resin powder                                                                              200    200    200  200                                    prepared in Preparation Example 1                                              Mineral spirits       20     60    180  250                                    Potassium oleate soap                                                                                2      3      4    5                                     Thickness of                                                                            faced surface   140     97   84   68                                  coating film                                                                            reverse surface  54     65   72   57                                  ______________________________________                                          .sup.*1 Coulomb yield is poor and the coating film obtained became             discolored.                                                              

The electrodeposition, drying and baking were carried out in the same manner as the foregoing Coating Example 1, and the thickness of the coating film as obtained was shown in Table 1.

COATING EXAMPLE 8 - 12

The preparations of electrodeposition bath were carried out in the same manner as the foregoing Coating Example 1, except that the amount of potassium oleate soap which is represented by weight parts as solid was as in the following Table 2. The electrodeposition was carried out in the same manner as the foregoing Coating Example 1.

                                      TABLE 2                                      __________________________________________________________________________     Coating Example No.                                                                               8   9   10  11  12                                          __________________________________________________________________________     The epoxy resin powder                                                         prepared in Preparation Example 1                                                                 200 200 200 200 200                                         Mineral spirits    100 100 100 100 100                                         Potassium oleate soap                                                                             0   0.5 2.0 8.0 15.0                                        Condition of bath  not                                                                            dis-                                                                           pers-                                                                              Good                                                                               Good                                                                               Good                                                                               Good                                                           ed                                                          Film appearance    --  Good                                                                               Good                                                                               .sup.*1                                                                            .sup.*2                                     __________________________________________________________________________      .sup.*1 The coating film obtained became slightly discolored.                  .sup.*2 The coating film obtained became discolored and had a large numbe      of craters.                                                              

PROPERTIES OF COATING FILM

The coating films on the faced electrode side obtained in Coating Examples 1 to 3 were tested in respect of the following properties. For comparison, the articles coated by the known electrostatic powder coating method and by using powdery particles obtained in Preparation Examples 1 to 3 were tested. The results were shown in the following Table 3.

                                      TABLE 3                                      __________________________________________________________________________     Type of vehicle                                                                              Epoxy resin   Polyester     Acrylic resin                        Coating method                                                                 and coating compositions                                                                     Electro-                                                                             Electrostatic                                                                          Electro-                                                                             Electrostatic                                                                          Electro-                                                                             Electrostatic                        methods deposition                                                                           coating deposition                                                                           coating deposition                                                                           coating                              of      (coating                                                                             (Preparation                                                                           (Coating                                                                             (Preparation                                                                           (coating                                                                             (Preparation                   Items test    Example 1                                                                            Example 1                                                                              Example 2                                                                            Example 2                                                                              Example 3                                                                            Example                        __________________________________________________________________________                                                     3                              Film                                                                           thickness                                                                            --      --    115μ --    75μ  --    100μ                        Film                                                                           appearance                                                                           --      Good  Good    Good  Good    Good  Good                           Gloss 60°specular                                                             gloss   80    81      88    95      84    93                             Hardness                                                                             Pencil                                                                         Hardness                                                                               2H    2H      H     H       2H    2H                             Impact                                                                               du Pont's                                                                resistance                                                                           Impact  No crack                                                                             No crack                                                                               No crack                                                                             No crack                                                                               No crack                                                                             No crack                             Tester  at 50 cm                                                                             at 50 cm                                                                               at 10 cm                                                                             at 10 cm                                                                               at 20 cm                                                                             at 30 cm                             (1/2"×500g)                                                        Moisture                                                                             200 hrs. at                                                              resistance                                                                           40° C,                                                                          No    No      Slightly                                                                             Slightly                                                                               No    No                                   100%    blistering                                                                           blistering                                                                             blistering                                                                           blistering                                                                             blistering                                                                           blistering                           reactive                                                                       humidity                                                                 Salt spray                                                                           ASTM B117-64                                                             resistance                                                                           for 200 hrs.                                                                           1.5 mm                                                                               1.0 mm  5.0 mm                                                                               5.0 mm  2.0 mm                                                                               2.0 mm                         __________________________________________________________________________ 

What is claimed is:
 1. A powder coating composition for electrodeposition which comprises powdery particles of a polyester resin powder coating composition having 0.1- 50 microns particle size wet with an organic liquid which does not dissolve the powdery particles and is nearly insoluble in an aqueous medium and which has a boiling point of 50°-250° C. in the range of 5-100 parts by weight per 100 parts by weight of the powdery particles, said wet powdery particles dispersed in an aqueous medium containing 0.05-5 parts by weight per 100 parts by weight of the powdery particles of a surface active agent.
 2. The powder coating composition of claims 1 wherein said aqueous medium is water or a mixture of water and a water soluble organic solvent.
 3. The powder coating composition of claim 1 wherein said powdery particles have 1-20 microns particle size and are wet with an organic liquid in the range of 20-60 parts by weight per 100 parts of the powdery particles.
 4. The powder coating composition of claim 1 wherein the powdery particles are of a polyester resin of ingredients comprising terephthalic acid, isophthalic acid, ethylene glycol and neopentyl glycol, the resin having an average particle size of 10-20 microns, the particles being wet with hexane, the aqueous medium comprising water and n-butyl alcohol, and the surface active agent being potassium oleate soap.
 5. A powder coating composition for electrodeposition according to claim 1, in which said polyester is wet with at least one liquid selected from the group consisting of hexane, heptane, isooctane, mineral spirit, naphtha, kerosene, amyl alcohol, cyclohexanol, hexyl alcohol, heptyl alcohol and octyl alcohol.
 6. A powder coating composition for electrodeposition according to claim 1, in which said water soluble organic solvent is at least one member selected from the group consisting of methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, propylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, acetone, diacetone, methylethyl ketone, ethyl acetate, propyl acetate, methyl acetate, amyl acetate, methyl cellosolve acetate and methyl carbitol acetate, and is used in the range of less than 90% by weight.
 7. A powder coating composition for electrodeposition according to claim 1, in which said powdery particles wet with organic solvent and a pigment are suspended in a range of about 5 to about 20% by weight into the aqueous medium.
 8. A powder coating composition for electrodeposition according to claim 1, in which said surface active agent is at least one member selected from the anionic surface active agents consisting of fatty acid alkyl metal salts, alkylsulfates, alkylsulfonates, alkylarylsulfonates and polyethylene glycol alkyl ether sulfonates, and the nonionic surface active agents consisting of polyoxyethylene alkyl ethers and polyoxyethylene alkyl esters.
 9. A powder coating composition for electrodeposition according to claim 1 in which said powdery particles wet with organic liquid is at a concentration of about 5 to about 20% by weight in the electrophoretic deposition bath. 